Recently, electronic devices become smaller, thinner and lighter rapidly, and particularly in the office automation field, computers are changed from desktop to laptop or notebook. In addition, as an electronic note and an electronic still camera appear, the conventional hard disk and floppy disk are small-sized and a memory card is also researched as a new small memory medium.
Together with such trends of the electronic devices, an electrochemical device for supplying power to the components is also required to have high performance, and a lithium secondary battery is well accorded with this requirement.
The lithium secondary battery is classified on the basis of components of its electrolyte into a lithium ion battery composed of a liquid electrolyte and a lithium polymer battery composed of a solid or gel-type polymer electrolyte, and these batteries use different kinds of membranes having different kinds of polymers and structures.
The lithium ion battery uses a micro-porous film made of polyolefin polymer selected from single polymer such as PE [poly(ethylene)] and PP [poly(propylene)], and copolymer such as ethylene/butene copolymer, ethylene/hexene copolymer and ethylene/methacrylate copolymer, as a membrane. The membrane has one layer, or multi layers like PE/PP or PP/PE/PP as necessary. However, since the polyolefin membrane has no affinity to electrolyte, it is difficult to make various kinds of batteries except a can-shaped battery due to leakage of the liquid electrolyte.
In order to overcome such a problem, a gel-type polymer electrolytic membrane having an affinity to the impregnated electrolyte is proposed.
The gel-type polymer electrolytic membrane is made from copolymer such as PVDF [poly(vinylidene fluoride)] and P(VDF-HFP) [poly(vinylidenefluoride)-co-(hexafluoropropylene)], or polymer such as PAN [poly(acrylonitrile)], PEO [poly(ethylene oxide)], acrylate by means of phase inversion, solution casting, in-situ crosslinking, hot melting or the like.
As a method for manufacturing PVDF gel-type polymer electrolyte using the phase inversion, a process technique of Bellcore Co. (now, its name is changed into Telcordia Technologies Co.) is representative. That is to say, U.S. Pat. No. 5,456,000 and U.S. Pat. No. 6,322,923 disclose a method for manufacturing a cell by making a polymer membrane made of P(VDF-HFP) copolymer added by DBP (dibuthyl phthalate) plasticizer, then inserting this membrane between anode and cathode and laminating them, then perforating the membrane by a plasticizer extracting process and drying it, and then injecting an electrolyte thereto under the anhydrous circumstance.
In addition, U.S. Pat. No. 5,429,891 discloses a method for making an electrolytic membrane by putting a silica inorganic filler and a crosslink agent such as trimethylolpropane trimethacrylate, trimethylolpropane triacrylate or glycerol trimethacrylate together into P(VDF-HFP) copolymer, then film-casting and drying it, and then irradiating an electronic beam thereto for 3-dimensional crosslinkage.
In addition, Korean Laid-open Patent Publication No. 2002-0069601 discloses a method for making a gel-type polymer electrolyte by dissolving PVDF or P(VDF-HFP) copolymer, porous silica and plasticizer into acetone, then coating it on a PET film and then drying, then extracting the plasticizer to form a porous polymer membrane, then coating or dropping a nytril thermosetting polymer on the polymer membrane to make a membrane, then positioning the membrane between cathode and anode, and then applying heat and pressure thereto so as to make the gel-type polymer electrolyte in which cathode/polymer membrane/anode are integrated.
However, the gel-type polymer electrolyte has low mechanical strength of the membrane, so it has a limit in increasing a process rate during the battery manufacturing procedure. In addition, the battery manufacturing procedure is complicated, particularly having an additional plasticizer extracting process, and the electrolyte is penetrated into the membrane ununiformly.
In order to solve these problems, there is proposed a multicomponent system complex polymer electrolytic membrane in which a porous polyolefin membrane is used as a strength support layer and a PVDF polymer is coated or laminated on one or both sides of the support layer as an electrolyte absorption layer.
For example, U.S. Pat. No. 5,681,357, U.S. Pat. No. 5,688,293 and U.S. Pat. No. 5,716,421 disclose a membrane manufacturing method in which a porous polyolefin membrane is dipped into or coated on a polymer composition to make a hybrid electrolyte, and then the polymer composition is polymerized by heat and pressure applied during a packaging process so as to be combined to electrodes. However, the heat and pressure applied during the packaging process may damage the package of the battery, and gas is generated from the composition during a curing process.
In addition, U.S. Pat. No. 5,853,916 discloses a multi-layer polymer gel-type electrolyte made by laminating a gel-type porous PVDF polymer layer and a non-gel micro-porous polymer layer. However, this gel-type porous membrane has a problem that it cannot increase a content ratio of electrolyte due to a low porosity.
In addition, Korean Laid-open Patent Publication No. 2000-0077418 discloses a method for improving a low mechanical strength of a polymer electrolyte by coating electrolyte on both cathode and anode, and then inserting a polyolefin membrane, that is a porous PE (polyethylene) film or a two-layer porous film composed of PE/PP (polyethylene/polypropylene), between them. However, since the gel-type electrolyte coated on the electrodes is sensitively reacted to moisture, the process should be progressed under an anhydrous circumstance. Moreover, since the gel-type electrolyte layer should be coated on both electrodes, it is difficult to manufacture a thin battery.
WO 99/54953 discloses a method in which a polymer suspension solution containing a lithium mixture in a mixed solvent of acetone and NMP (n-methyl pyrrolidone), such as a PEO (polyethylene oxide) suspension solution containing lithium salts such as LiPF6 (lithiumphosphohexafluoride), a PVDF (polyvinylidene fluoride) suspension solution containing lithium salts or a PMMA (polymethyl methacrylate) suspension solution containing lithium salts, is coated on a micro-porous PP (polypropylene) film or a multi-layer micro-porous laminate film composed of PP/PE/PP and then dried to make a complex polymer electrolyte, and then the made complex polymer electrolyte membrane is positioned between anode and cathode and then dipped in an organic electrolyte solution to finally manufacture a battery. However, since the coated polymer layer in the complex polymer electrolyte has a granular structure, it takes a long time to absorb the organic electrolyte, and an ununiform gel-type layer is formed due to the dipping process.
As mentioned above, the conventional complex multi-component membrane configured by coating a PVDF gel-type polymer layer on a strength support layer or laminating or uniting it in a film shape shows a low absorption rate during injection of electrolyte since the coated electrolyte absorption layer has no internal pore, and a battery performance is deteriorated due to ununiform absorption of electrolyte.